1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, the present invention relates to the art of sealing an electrochemical cell or battery and, still more particularly, to hermetically sealing the electrolyte charging or fill opening in an electrochemical cell by means of a unitary sealing member. The unitary sealing member of the present invention is first force-fit into sealing registry with the electrolyte fill opening to provide a secondary seal that prevents out gassed by-products from compromising the primary, hermetic weld between the sealing member and the cell casing.
2. Prior Art
Leakage of electrolyte and gases from an electrochemical cell caused by a compromised seal is extremely undesirable and can even be fatal when the cell is used as the power source for an implantable medical device and the like. In electrochemical cells having a metal casing, one means of providing a hermetic seal for an electrolyte fill opening and the like is by welding a seal member in the casing. However, the casing proximate the weld conducts heat to the electrolyte contained therein and some electrolyte evaporation invariably occurs. When these gases escape from the cell casing they are referred to as out gassed by-products and such escaping gases leave pill holes in the weld thereby compromising hermeticity.
U.S. Pat. No. 5,004,656 to Sato et al. discloses a flat-type battery having a hermetically sealed electrolyte charging opening. The sealing member consists of a spherically-shaped body press-fit into sealing registry with the charging opening followed by a disc-shaped plate covering the sealing body and welded to the battery casing. The spherically-shaped sealing body and the disc-shaped plate can be separate members or combined as an integral unit. In any event, this prior art patent discloses that the press fit sealing body prevents compromise of the weld between the disc-shaped plate and the battery casing caused by escaping out gassed by-products.
Thus, the prior art recognizes that it is extremely important for a hermetically sealed cell to retain its manufactured, gas tight condition, even after prolonged periods of storage or use. This is nowhere more important than in electrochemical cells used in implantable medical devices and the like. Sato et al. and others have recognized that simply welding a sealing means over a cell opening does not necessarily assure gas tight hermeticity. Instead a secondary seal is required to prevent evaporated electrolyte gases from escaping and causing the formation of pin holes in the primary weld. However, it is also desirable to minimize the size of such cells by reducing their volume and to simplify the manufacturing and assembly processes by reducing the number of cell components. Furthermore, the seal member should be confined within the perimeter of the electrolyte fill opening and not protrude beyond the outer surface of the cell casing to provide the casing with an aesthetic profile. Therefore, in addition to providing a hermetically sealed fluid filled container, uncompromised by out gassed by-products, a primary object of the present invention is that the seal does not hinder the insertion of the container, for example, a cell or battery, into a receptacle for the cell in an implantable medical device and the like, where a premium is placed on economy of size.